In the past, telephone line access was controlled by mechanical means. For example, for conventional dial telephones, a mechanical lock was designed to prevent the dial from turning; and in the cases of the tone-type telephone, a lock was provided which prevents access to the buttons.
With the introduction of the modular telephone receiver actuated by electronic dial pulse dialing, the mechanical type dial locks became useless. The result is that in the case of a large number of telephone systems, the addition of security means is impossible using existing apparatus. In addition, there is an increasing number of cordless telephones which provide telephone line access via the air waves, and for which security presents a special problem. For such telephones, the system must be securable without subverting the entire purpose of cordless operation, which is to greatly reduce the necessity for being in physical contact with the base unit.
The principal object of the present invention is to provide an inexpensive, self-contained, line-powered modular means for providing security in an existing telephone system, regardless of the type of telephone unit being used with the system, whether it be dial pulse, tone-type or cordless.
In accordance with the present invention, a security circuit is interposed between the standard modular telephone line receptacle and the base telephone unit's line receptacle whether the latter is of the cordless type, or of the conventional wire-connected type. In either case, the interposed Security Unit of the present invention consists of three parts. These are an Access Limiter and Bypass Circuit, a Ring Detector Circuit, and a Mechanical or Electronically Actuated Enabling Device. The Access Limiter and Bypass Circuit, which is connected in series with the telephone line, allows the telephone to operate without signaling an off-hook condition. Since the telephone is thus enabled to operate, it functions to provide input to the Electronically Actuated Access Enabling Device, and may condition operation of the Mechanically Actuated Access Enabling Device.
A particular feature of a telephone system which includes one of the Security Units of the present invention is that it is receptive to incoming calls, even though the Access Enabling Device has closed the line to outgoing calls. Another feature is that it is line-powered, in that no batteries are needed.
The Ring Detector Circuit is connected to monitor the presence of an incoming telephone call. When the voltage across the telephone line increases above a preselected voltage V.sub.1, which is 48 volts in the standard telephone line, a telephone call is detected. The Ring Detector Circuit becomes conducting, and allows the incoming signal to pass to the telephone. The Ring Detector Circuit is constructed to allow the enabling voltage to remain a short period, say five seconds, after the ringing stops. Thus, the incoming call can be answered any time during the ring cycle and for a short period thereafter. When the telephone line is in on-hook condition, the line voltage is below the voltage V.sub.1 at which the Ring Detector Circuit becomes conducting. The entire device presents no load to the telephone line in the on-hook condition.
The Access Limiter and Bypass Circuit functions so that when the telephone is in off-hook condition, the telephone cannot draw sufficient current from the telephone line to signal an off-hook condition; thus the line is secure.
When the Access Enabling Device of the present invention is actuated, the line is opened between the telephone instrument and the telephone line for outgoing calls; otherwise, outgoing calls are blocked.
In a preferred embodiment of the present invention, developed especially for cordless type telephones, the Access Enabling Device takes the form of a Sequential Pulse-Actuated Electronic Circuit which, when it receives and identifies the proper sequence of pulses, sends a signal to bypass the Access Limiting Circuit, permitting access to the telephone line for outgoing calls. The Access Enabling Device, in a preferred example described in detail hereinafter, is responsive to a preselected sequence of pulses equivalent to, for example, three digits, to send a signal which causes the Access Limiter to be bypassed, thereby allowing outgoing calls access to the telephone line.
In a preferred form of the invention, the Access Enabling Device, which functions to receive and identify the preselected pulse sequence and to signal to bypass the Access Limiting Circuit, comprises five components in circuit combination.
These include the following:
An Access Pulse Detector and Shaper is connected to receive the pulsed signals generated by using the telephone set. A Timing and Logic Circuit is connected to receive the output from the Access Pulse Detector and Shaper and to control operation of a Pulse Counter and Sequential Latch and Counter Circuit. The Sequential Latch and Counter Circuit is programmed to detect a specific sequence of pulses by means of the Data Distributor Circuit. Upon detection of a specific sequence of pulses, the Sequential Latch and Counter Circuit will issue an enable signal that provides complete telephone line access for outgoing calls.
In a modified, simplified form of the invention, the Electronic Access Enabling Circuit is replaced by a key-actuated switch, which is mechanically actuated to open and close the line to outgoing calls.